Natural resources such as gas, oil, and water residing in a subterranean formation or zone are usually recovered by drilling a wellbore down to the subterranean formation while circulating a drilling fluid (also known as a drilling mud) through the drill pipe and the drill bit and upwardly through the wellbore to the surface. The drilling fluid serves to lubricate the drill bit and carry drill cuttings back to the surface. After the wellbore is drilled to the desired depth, the drill pipe and drill bit are typically withdrawn from the wellbore while the drilling fluid is left in the wellbore to provide hydrostatic pressure on the formation penetrated by the wellbore and thereby prevent formation fluids from flowing into the wellbore.
The next operation in completing the wellbore usually involves running a string of pipe, e.g., casing, in the wellbore. Primary cementing is then typically performed whereby a cement slurry is pumped down through the string of pipe and into the annulus between the string of pipe and the walls of the wellbore to allow the cement slurry to set into a hard mass (i.e., sheath), and thereby seal the annulus. The cement slurry ideally displaces the drilling fluid from the annulus. However, certain cement slurries are often incompatible with the components in the drilling fluid. For example, it is known in the art that when a cement slurry containing free polyvalent metal cations, especially calcium, are brought into contact with a drilling fluid containing clay or certain polymers, a highly viscous plug can form near the interface of the drilling fluid and cement slurry, creating problems well known in the art. Also, high density drilling fluids commonly contain lignins as dispersants that can lead to excessive retardation in cement slurries. To overcome such problems, a technique has been developed in which a spacer fluid is injected into the wellbore between the drilling fluid and the cement slurry. The spacer fluid is usually compatible with both types of fluids, and it has a density sufficient to displace the drilling fluid from the wellbore.
During drilling, the wellbore may experience washout in which its hole size becomes enlarged. As a result, the actual size of the annulus may be unknown by the time the cement slurry is pumped therein, making it difficult to know when a sufficient amount of cement slurry to fill the annulus has been pumped into the wellbore. One way that can be used to determine the appropriate time to stop pumping the cement slurry into the wellbore is to identify when the cement slurry returns to the surface of the earth. However, this identification has proven to be a challenge, particularly when performing drilling offshore where cement returns to the sea floor are extremely difficult to confirm. While attempts have been made to recognize such cement returns by placing dyes in the cement shiny, those attempts often have failed. Therefore, a need exists to develop a method for determining the amount of cement slurry required to fill the annulus of a wellbore.